A day after Rachel Frederickson won the latest US season of The Biggest Loser, after shedding nearly 60 per cent of her body weight, attention wasn't focused on her $US250,000 ($A281,357) win — but rather the criticism surrounding her loss.

Experts cautioned that regardless of her current weight, the criticism being levied on social media about her losing too much isn't helpful.

Biggest Loser Australia trainer Shannan Ponton has also weighed in on the controversy, saying he was shocked by Frederickson's dramatic weight loss.

''I was personally shocked,'' he told AAP. ''That's a hell of a lot of weight to lose.''

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Some took to Twitter to call for a minimum body mass index (BMI) threshold to be introduced, while one even quipped the victory confetti might knock Frederickson over.

Frederickson was a three-time state champion swimmer at Stillwater Area High School in Minnesota, then turned to sweets for solace after a failed romance with a foreign exchange student she followed to his native Germany.

Before and after: Rachel Frederickson lost 60 per cent of her body weight.

Frederickson's newly thin frame lit up Twitter on Wednesday, with many viewers pointing to the surprised expressions on the faces of trainers Jillian Michaels and Bob Harper during the show's Tuesday night finale.

Many tweeted that Frederickson looked anorexic and unhealthy, while others congratulated her for dropping 155 pounds (70 kg).

Local participants are also subject to psychological screening before they sign up for the popular reality show. But asked whether such tests scrutinise a contestant's chances of developing an eating disorder, he said possible contestants usually already have one.

''I think most of our contestants have an eating disorder - an overeating disorder,'' he said.

In his professional life, Ponton said he's never witnessed someone who compulsively overeats swing to the other extreme of disorders, such as anorexia.

Lampert said a more constructive message to send young people would centre on well-rounded health and the importance of eating well, moving well and sleeping well.

"We certainly see a lot of people who struggle with eating disorders who use the same behaviours on that show to an extreme," she said.

"That can't be helpful."

Joanne Ikeda, a dietitian and retired faculty member at the University of California at Berkeley's Department of Nutritional Sciences, added that focus needs to be on embracing body-size diversity.